Allow to specify project in build endpoint of webhook proxy

[michaelsauter]

The webhook proxy allows (with a minor exception for the prov app) only to manage/proxy within it's own project. This is good for security, but when Rundeck is gone, we need to access (at least read from) the opendevstack namespace as well.

PR #227 added a hack to master to allow building any URI. That should be reverted, and instead we should allow to build only for the current namespace and whatever is configured via a new env var ALLOWED_EXTERNAL_PROJECTS, which, if not set, defaults to opendevstack. The build endpoint payload then has to learn a new field, project, which allows to specify a project where the repository is located.

@kiwo I'll implement this in the next few days, you'll need to update your PR to the provisioning app then.